A The present invention relates to sheet-fed printing machines and, more particularly to an improved stack changing device for sheet feeders of sheet-fed printing machines.
It is a known practice, in sheet feeders of sheet printing presses, to provide arrangements for automated stack change. These may consist of rack-type structures, so-called remaining-stack carrying devices, which are provided with thrusting and lifting drives for the horizontal and vertical movement. Such so-called non-stop stack changers are suited, during the printing of paper sheets, i.e. in machine running, to remove stack remainders of worked-out sheet stacks from a pallet provided, for example, with grooves, and to deposit them again on a new sheet stack subsequently installed in the sheet feeder. Known devices are distinguished by high constructive and technical expenditure and require special constructions of the sheet feeders. Further, devices are used of which the remaining-stack carrying device(s) have a rack engaging into the grooves of the pallet. In the unification of the remaining stack with newly installed main stack, this rack has to be removed as a whole between the two stack parts. This involves high drive forces and places great strain on the next-lying sheets of the stack. Furthermore, retaining means are to be provided that prevent a shifting of the stack parts, and, in so doing strongly stress the stack edges. Furthermore, the operation of the sheet feeder itself is severely hampered or even made impossible. The sheet flow is difficult to control in the changing process, so that waste sheets result again and again.
Devices have already been developed that avoid in part some of the disadvantages described.
Thus, from DE 393 1710 C2 there is known a nonstop sheet feeder for rotary sheet-fed machines. It has a remaining-stack carrying device which is arranged underneath a conveyor table leading from the sheet feeder to the sheet-fed rotary machine. The remaining-stack carrying device has a closed frame on which there are arranged nonstop rods which can be driven as piston rods of individual cylinders by means of a pressure medium, and which are drivable into grooves of a pallet carrying a sheet stack. The nonstop rods lie, in the driven-in state, on both sides of the frame and are to be removed in succession out of the range of the sheet feeder. While the rods have individual drive cylinders, the patent discloses nothing about the sequence of operation. The arrangement under the sheet feeding table is unsuitable for practical operation.
A sheet feeder is known From DE 4 203 500 A1. It has, parallel to the sheet feeder and allocated to this on the face side, a remaining-stack carrying device as an independent component. Then, over a common drive, there are provided individually drivable pointed bars which are drivable into grooves of a pallet carrying a sheet stack. The drive has individual chain gears which are couplable to the respective pointed bars. For the guiding and accessibility of the chain gears, special constructive measures are required. The chain gears completely block the space in front of the sheet feeder, so that the latter is not accessible. Further, from DAS 1095297 there is known a sheet feeder with several stack-lifting mechanisms. This feeder has a fork-shaped remaining-stack carrying device which is provided with remaining-stack bars slidable into grooves of a pallet. The device makes possible the taking over of a remainder of a sheet stack from the pallet, for the continuous feeding-in of the sheets while a new sheet stack is installed into the sheet feeder. The remaining-stack lifting mechanism is connected with a separate lifting mechanism parallel to the main lifting mechanism within the sheet feeder, so that the remaining stack is continuously liftable. The operating range of the remaining-stack carrying device is very severely restricted by the lever arrangement, so that only short time intervals are available for the stack changing. The remaining-stack carrying device hampers access to the sheet feeder, since it has to be placed outside of the sheet feeder as long it is needed.
In view of the foregoing, an object of the present invention is to provide an improved stack changing device which overcomes the problems associated with prior art designs.
A more specific object of the present invention is to provide a stack changing device which allows for simple, continuous and efficient stack changing along with improved operational parameters.
The present invention provides these and other advantages and overcomes the drawbacks of the prior art by providing a stack changing device which utilizes an improved method and device for receiving a remaining-stack and transferring the remaining stack to a newly fed-in sheet stack.
It is advantageous that carrying and spacing bars movable independently from one another are provided in the device, which are pulled out of the stack zone not simultaneously but intermittently for the unburdening of the sheet material. The arrangement of the remaining-stack bars in a remaining-stack carrying device of the type described makes possible access to the operating zone also during the changing process. Specifically, an operating space is kept free in the zone of the bar guidance during the stack changing.
Through the fact that the remaining-stack bars are formed of different length according to type, there results an improved working process. In the changing operation, in particular, the time for the sliding-in and pulling-out of the remaining-stack bars can be reduced by the means that the remaining-stack bars are already driven into a waiting position before the changing, and are slid in completely only at the beginning of the changing process.